1. Field of the Invention
The present invention relates to an asymmetric aperture diaphragm placing structure for a projection lens, which secures an asymmetric aperture diaphragm having an aperture asymmetrical about the optical axis of the projection lens to a lens holder such as a lens chamber holding the projection lens in a projection type image display apparatus comprising image display means such as digital micromirror device (hereinafter referred to as “DMD”), for example; and a projection type image display apparatus comprising such an asymmetric aperture diaphragm placing structure.
2. Description of the Prior Art
A projection type image display apparatus has conventionally been known, in which a DMD comprising a number of mirror elements with variable light reflection angles adapted to change reflection angles of illumination light according to image signals so as to reflect only the signal light required for forming an image toward a projection lens system is used as image display means (light valve). The DMD comprises rectangular minute mirrors (mirror elements) having a high reflectance adapted to change their inclinations within a predetermined angle range according to image signals, which are formed on a silicon memory chip by using the CMOS semiconductor technology. The projection type image display apparatus using the DMD is configured so as to regulate the reflecting direction of light from the light source by changing the angles of mirror elements, such that only desirable reflected light is converged onto a screen, so as to project an image.
Thus, the DMD has such a characteristic that, of the light obliquely incident on its mirror element surface, light to become signal light is emitted toward the projection lens whereas light (unnecessary light) not to become signal light is emitted into a direction not oriented to the projection lens. However, a part of the light to become unnecessary light may be scattered by the mirror element surface, and thus formed scattering light may enter the projection lens and lower the contrast. Therefore, in order to eliminate influences of scattering light and improve the contrast, projection type image display apparatus using the DMD are usually provided with a diaphragm member by which the passing area of a luminous flux which can be made incident on the projection lens is restricted to a predetermined range. Since the DMD essentially has characteristics of oblique incidence and oblique emission, the aperture of the diaphragm member (diaphragm aperture) has been known to have a form asymmetrical about the optical axis of the projection lens, i.e., such a form that the diaphragm aperture cannot completely coincide with the original form after the diaphragm member is rotated about the optical axis by an angle of 2π/N (N=2, 3, 4, . . . ) or reversed (see TEXAS INSTRUMENTS INCORPORATED, Application Report LDPA006A-JUNE2001 “DLP Projector System Optics Contrast-Enhancement Techniques”).
An asymmetric aperture diaphragm whose diaphragm aperture does not have a circular form though not completely asymmetrical about the optical axis may be used outside of the projection type image display apparatus using the DMD (see Japanese Unexamined Patent Publication No. SHO 63-144319, Japanese Unexamined Patent Publication No. HEI 06-011681, and Japanese Patent Publication No. 3137435). However, structures for attaching such an asymmetric aperture diaphragm to a lens chamber or the like, i.e., asymmetric aperture diaphragm placing structures, have not been disclosed yet.
As mentioned above, an asymmetric aperture diaphragm may be placed in a lens chamber for a projection lens in a projection type image display apparatus using a DMD. When the diaphragm aperture of the asymmetric aperture diaphragm has a form asymmetrical about the optical axis, however, the orientation of the asymmetric aperture diaphragm at the time of placement is limited to a predetermined direction. If the asymmetric aperture diaphragm is placed in other directions, the diaphragm member may lose its aimed optical performances.
It has conventionally been known to provide the lens chamber and asymmetric aperture diaphragm in the projection lens with marks indicating the direction in which the asymmetric aperture diaphragm is placed, so as to take notice that the placing direction should not be mistaken when placing the asymmetric aperture diaphragm. However, this has not been sufficient as means for always placing the asymmetric aperture diaphragm into a correct direction.